I think it has as much to do with the size of prey as it does with the livliness of it. Think about it...a snake comes accross a mouse's nest. Is it really going to constrict the pinkies one by one, or just eat them like popcorn? Now, if mom comes home while the snake is still there, you better believe there's going to be some serious squeezing going on!
Some of my juveniles that never constricted pinkies or fuzzies, squeeze all their food items now that they're eating hoppers, weanlings, and adult mice. That's why I think it has to do with an ingrained reaction tot he size/type of the prey.
Just a thought.